This invention relates to a novel process for the reduction of metal halates in aqueous alkaline solutions. More specifically, the invention relates to a process for decreasing the amount of metal bromate in aqueous alkaline solutions and recovering valuable metal bromide from such aqueous alkaline solutions. For example, in a scrubber system for the removal of halogens from process vent streams, an aqueous alkaline solution is generally used as a scrubbing medium. Minimizing the bromate content allows the use of low cost materials of construction, eliminates the potential for release of hazardous materials and achieves high bromide purity in the recovered product.
The potential to form halates is known. The literature reports the formation of halate when a halogen and a basic metal compound react. Metal halides result when a basic metal compound and a reducing agent react with a halogen. Ammonium halide results when ammonia reacts with a halogen.
The preparation of metal halide from a basic metal compound and a halogen in the presence of a reducing agent is well known in the art. Acidic conditions are reported, and the reaction conditions are such as to inhibit the formation of halates. In the absence of a reducing agent, a halogen compound reacts with a metal base in two steps, forming the hypohalide, which then tends to decompose to the halate. For example, U. S. Pat. No. 2,828,184 reports: EQU 3 I.sub.2 +6 KOH.fwdarw.3 KI+3 KIO+3 H.sub.2 O (1) EQU 3 KIO.fwdarw.KIO.sub.3 +2 KI (2)
However, when a reducing agent is included in the reaction with the halogen, formation of the halate is prevented.
U.S. Pat. No. 4,514,374 discloses preparation of aqueous metal bromide solution without the concurrent production of bromate by reaction of calcium hydroxide, bromine and methanol in aqueous solution. The reaction is depicted as follows: EQU 3 Ca(OH).sub.2 +3 Br.sub.2 +CH.sub.3 OH.fwdarw.3 CaBr.sub.2 +CO.sub.2 +5 H.sub.2 O (3)
In this process, only a stoichiometric amount of base is used, not excess base nor strongly alkaline reaction conditions.
Other patent literature cites the use of ammonia, ammonium ion, or amines to form the halide directly from halogen. For example, the reaction with NH.sub.3 can be written as follows: EQU 3 Br.sub.2 +8 NH.sub.3 .fwdarw.6 NH.sub.4 Br+N.sub.2 ( 4)
This reaction sequence is not without significant drawbacks including: 1) formation of explosive compounds such as halo amines as a by-product; and 2) significant loss of ammonia to vent streams.
Numerous examples of reducing agents are known in the prior art. For example, U.S. Pat. No. 2,269,733 discloses the use of ammonia, ammonium hydroxide, ammonium bromide, urea, formic acid, oxalic acid, ammonium carbonate, ammonium bicarbonate, and formamide as well as other metal oxides, hydroxides and carbonates. However, a small amount of free bromine is maintained during substantially the entire course of the reaction to form sodium bromide while the pH of the reaction is maintained below pH 7.
In U.S. Pat. No. 3,431,068 there is disclosed a method of preparing alkali metal halides by reacting an alkali metal hydroxide with an elemental halogen in a liquid, saturated aliphatic or alicyclic alcohol or ketone, or a saturated aliphatic aldehyde. According to this process, the formation of unwanted halate salt by-product associated with alkali metal halide production is diminished or eliminated. It is also disclosed that the use of nitrogen containing compounds such as NH.sub.4 Br can result in explosive mixtures. The alcohol, aldehyde and ketone mentioned in this reference are not employed as reducing agents but as solvents and are present in large excess. Use of alcohols or ketones as reaction medium for scrubbing vent streams is prohibited by the emission losses that would occur.
U.S. Pat. No. 4,083,942 discloses the use of formic acid as a reactant. The process is illustrated by the following equation: EQU Ca(OH).sub.2 +HCOOH+Br.sub.2 .fwdarw.CaBr.sub.2 +CO.sub.2 +2 H.sub.2 O(5)
In the foregoing process, bromine and an alkaline compound are alternatively and incrementally added to an aqueous mixture of formic acid and a less than equivalent amount of metal compound, while maintaining the pH below 7.0.
U.S. Pat. No. 3,592,600 relates a process for the recovery of bromine and/or iodine from the reaction products of oxydehydrogenations of hydrocarbons in which bromine and/or iodine serves as a catalyst. The reaction product leaving the reactor, primarily hydrogen halide with some elementary halogen, is treated with an aqueous ammonia solution to which has been added an amount of hydrazine sufficient for the reduction of the elementary halogen. The aqueous phase is separated and the by-products containing bromine and/or iodine separated in the working up of the organic phase are burned with oxygen containing gases and the gas from the combustion is scrubbed with an ammonia solution containing hydrazine. However, this process also suffers the drawback mentioned above wherein explosive halo amines are formed.
Amine halide is prepared by reaction of an amine with a metal base and a halide in solvent. For example, lithium carbonate, hydrazine and iodine react in water, as is disclosed by U.S. Pat. No. 4,111,991. Reaction conditions are maintained at pH 4.5 to 7.0.
Japanese Patent. No. 47-3399 recites reaction of Br.sub.2 with aqueous NaOH to generate sodium bromide and sodium bromate, as in reactions (1) and (2) above. Excess bromine is used in the reaction with caustic, leaving the pH of the solution at less than 7.0. The resulting bromate is then reduced using sulfurous anhydride, stannous chloride, metallic zinc, formic acid, hydroxylamine, hydroquinone, phenylhydrazine, hydrated hydrazine or the like. However, SO.sub.4, Cl, etc., or impurities generated in the reduction may be admixed and their removal is difficult resulting in products with extremely low purity. This reference also teaches the treating of a mixed solution of sodium bromide and sodium bromate with formic acid in an amount equal to or less than the theoretical amount needed to reduce the sodium bromate at less than 40.degree. C. Then, hydrogenated hydrazine is added to bring the solution to a pH of 1 or lower. The solution is subsequently concentrated, neutralized, filtered and heated to about 120.degree. C. While this procedure may help prevent the unwanted formation of sodium formate, the strongly acidic conditions which it employs prevents its use in alkaline scrubber systems such as those taught in this invention.
In U.S. Pat. No. 4,248,850, it was disclosed that metal bromides could be prepared by contacting in an aqueous medium a basic metal compound and bromine in the presence of added formaldehyde as a reducing agent. However, this method suffers from the draw-back of leaving unreacted formaldehyde in the product mixture which is difficult to remove. Also, acidic reaction conditions are used.
Thus the prior art for limiting halate accumulation falls into two classes: 1) metal hydroxides and a reducing agent in neutral to acidic conditions reacting with halogen whereby the formation of halate is inhibited, and 2) ammonia, ammonium ion, or amines reacting with halogen or halate to form halide. There remains a need to reduce halates under strongly basic conditions, for example at pH&gt;10 and preferably at pH&gt;13, whereby valuable by-products are recovered and the formation of explosive compounds is inhibited.
The strongly basic reduction conditions are important in certain operations. For example, in scrubber systems for scrubbing bromine from process vent streams, caustic solution containing sodium bromide are maintained for long periods of time. Sodium bromate tends to accumulate in the scrubber solution. In such systems, if bromate content is allowed to accumulate, the solution becomes very corrosive. Also, if spent scrubber solution is disposed of, sodium bromate creates a disposal hazard since acidification will release bromine and any uncontrolled reaction with certain compounds, such as amines, could lead to explosive mixtures. When using a reducing agent such as methanol in a scrubber system for removing halogens from process vent streams, it is desirable to use less than a stoichiometric amount of methanol to reduce the halates and to perform such reduction with the vent streams diverted from the scrubber. This reduces the potential to emit methanol in the vent gases, and to form methyl bromide.
It is therefore an object of this invention to reduce metal halates to metal halides in solutions which are strongly basic. It is another object of this invention to reduce the amount of corrosive bromate produced from the reaction of bromine and metal hydroxide or metal carbonate in vent streams by adding a reducing agent to a scrubber solution. It is a further object of this invention to recover substantially pure metal halide or metal halide solution from such scrubber solutions in order to conserve valuable raw materials.